16 BULLETIN 376, TJ. S. DEPARTMENT OF AGRICULTURE. 
on pipes up to 6 feet in diameter were abstracted, but no other than 
the writer's records are available for pipes between 6 and 12 feet in 
diameter. 
EQUIPMENT AND METHODS EMPLOYED FOR COLLECTING AND INTER- 
PRETING FIELD DATA. 
In order to correctly weigh any new data bearing on hydraulic 
formulas it is necessary to know in detail the equipment used and the 
steps pursued in both the field and office. Consequently these 
features are described in some detail. 
EQUIPMENT. 
Tapes. — High-grade steel tapes, graduated in feet and hundredths, 
were used in the determination of diameters, circumferences, etc. 
For distance chaining the tape was graduated to tenths. 
Level. — An 18-inch Berger engineer's wye level, equipped with a 
bubble whose sensibility was rated at 10 seconds of arc for 1 division 
of scale equal to one-tenth of an inch was used. The bubble vial was 
6.5 inches long; the telescope power was 35 diameters. The instru- 
ment was kept in excellent adjustment. 
With one exception the levels in these tests were closed within the 
limits suggested by the U. S. Geological Survey, the allowable error 
in feet being 0.017 Vdistance in miles. 1 The exception noted occurred 
in connection with the tests on pipe No. 37, where the levels were run 
in high wind, over deep sand. Several trials were made, but the best 
closure was to 0.023 foot, while to conform to the formula it should 
have been to 0.012 foot, the distance being about 2,500 feet. 
Rod. — A new Philadelphia rod, in three sections, equipped with rod 
level and vernier reading to thousandths of a foot was used in the 
determination of the elevations of gauge zeros with regard to an 
assumed datum. 
Thermometers. — Temperatures of air and water were taken with 
all-glass laboratory thermometers, graduated to degrees and fifths, 
Centigrade scale. The range covered in the graduations was only 
that liable to be encountered in the tests, so that each degree was 
represented by about three-sixteenths inch. 
Hydrometer. — Specific gravity of water in the pipes was tested by 
means of a laboratory hydrometer simultaneously with a like deter- 
mination of the temperature of the water. The hydrometer was 
afterwards tested by the U. S. Bureau of Standards. The proper 
corrections were thereafter applied to readings before computation was 
undertaken. 
Current meter. — A small Price cup current meter of the combina- 
tion type was used. This meter had been carefully rated by the U. S. 
i Precise Leveling, in Topographic Instructions of the U. S, Geological Survey, 1913, p. 100, 
